Protective glazing is used in many interior and exterior building applications, including windows, glass partitions, doors, etc., for safety, impact resistance and fire resistance. Protective glazing usually is a laminated structure consisting of a sandwich of several sheets of glass or polymeric panels bonded together by means of an interlayer of a polymer film placed between the sheets or panels. One or more of the glass sheets may be replaced by optically clear rigid polymer sheets, such as sheets of polycarbonate polymer. The interlayer is made of a relatively thick polymer film exhibiting a toughness and bondability as will cause the glass to adhere to the interlayer in the event of its being cracked or crashed. Protective glazing made using current interlayer films (e.g., polyvinylbutyrate (PVB)), used in windows, doors and partitions, must be sealed from the atmosphere for it to possess any fire resistance which would prevent the cracking or shattering of the glass in the event of a fire. In order to prevent the spread of fire, fire resistant materials which include steel and other opaque materials, and intumescent materials have been incorporated in doors, windows, and partitions. However, these materials are heavy and do not yield the optical clarity necessary for the visually transparent glazing applications.
U.S. Pat. No. 5,244,709 describes a technique in which an intumescent material, typically a hydrated alkali metal silicate film, is laminated between two panels of vitreous glass of differing thicknesses. This technique requires that both the film and the vitreous glass be fairly thick (0.5-5.0 mm and 8.0-21.0 mm respectively) to give the necessary fire protection and that once the localized heat limit is reached for the film, whether by fire or other means, the material intumesces, thus rendering it useless as a optical laminate. In many cases, additives are incorporated to reduce the temperature at which intumescence occurs. These additives may, in addition to lowering the intumescence temperature, increase both the haze and the cost of the final material.
U.S. Pat. No. 4,978,405 describes a method of manufacturing fire resistant protective glazing by incorporating a wire mesh into a film of methacrylate resin and other additives, and laminating this combined film between glass panels. The addition of the wire mesh, while giving some increased safety features, will influence and most probably lower the optical quality of the final glass product. The addition of additives to the methacrylate resin to increase the fire resistance, will also serve to lower the light transmittance and mechanical properties, create unacceptable amounts of haze and raise the cost of the final product. In addition, incorporation of the wire mesh into the film creates an aesthetically undesirable effect and increases the weight and mass of the final firescreening glass panel, making the panels more difficult to manipulate.
In U.S. Pat. No. 4,681,810, a sophisticated formulation including a char forming organic phosphate and an oxygen sequestering organic phosphite is added to PVB to increase its fire resistance. The addition of a high load of additives to the costly PVB film increases the final cost of the material beyond a commercially acceptable point for many applications.
A laminate of fluorocarbon resin interlayer films between specially formulated glass panels is described in U.S. Pat. No. 5,230,954. Fluorocarbon resins: specifically fluorinated ethylene propylene copolymer (FEP); tetrafluoroethlylene perfluoroalkoxy ethylene copolymer (PFA); polychlorotrifluoroethylene (PCFE); ethylene tetrafluoroethylene copolymer (ETFE); and polyvinylidene fluoride (PVDF); are thermocompression bonded to glass at a pressure of 12 Kg/cm.sup.2 at a temperature of 330.degree. C. Due to the high temperatures and pressures required in this technique, specially formulated glass and a high temperature autoclave are required to alleviate the possibility of cracking and breaking of the glass during lamination.
Each of these prior art protective glazing laminates: a laminate of intumescent material between vitreous glass; a laminate of wire mesh between methacrylate resin/additive laden films and glass sheets; high loads of fire resistive additives in PVB film laminates; and laminates of fluorocarbon resins adhered at high temperatures and pressures onto specially formulated glass; possess significant disadvantages that are inherent in the construction and manufacture of the laminate.
It is an object of this invention to provide a fire resistant fluoropolymer interlayer film suitable for use in protective glazing laminates produced under standard industrial laminating temperatures and techniques. This film has excellent fire resistance and optical qualities and high mechanical strength. These films may be formulated to be transparent, semi-opaque, or opaque, depending on their particular application. The films may incorporate a fiber reinforcement layer to increase structural strength of the laminate.